This invention relates to the launching and receiving of electromagnetic waves that are guided by and travel along a single conductor. More specifically, this invention relates to surface wave launchers of the type that form a transition between a coaxial cable and a surface wave transmission line.
As is known in the art, broadband, low-loss transmission of RF electromagnetic energy can be achieved through the use of a single conductor that is configured or treated to concentrate and confine the electromagnetic energy to a cylindrical volume that coaxially surrounds the conductor. This type of transmission line is known as a surface wave transmission line, a Goubau line, or G-line. In the more commonly known surface wave transmission lines, a conductor is surrounded by a coating of low-loss, dielectric. Since the phase velocity of electromagnetic energy that propagates through the layer of dielectric material is less than the free-space phase velocity, at least the majority of the electromagnetic energy is confined to the dielectric and a cylindrical volume of space that concentrically surrounds the dielectric coating. Other techniques for suitably decreasing the phase velocity of the transmitted signal also are known. For example, crimping an uncoated wire or machining threadlike grooves in the wire surface will cause a reduction in the phase velocity of signals traveling along the wire, thereby causing the uncoated wire to act as a surface wave transmission line.
In most systems that utilize surface wave transmission lines, the lines are utilized in combination with more conventional signal transmission structure such as coaxial cable and/or waveguide. In this regard, conventional equipment for generating and receiving signals is adapted for use with more conventional transmission structure such as coaxial cable or waveguide. Thus, transitions are required to couple signals between a surface wave transmission line and other transmission structure. Further, in many situations, use of only a surface wave transmission line is impractical. Specifically, bends and other discontinuities in a surface wave transmission line cause radiation of a portion of the electromagnetic energy traveling along the line, thereby resulting in transmission losses.
Systems in which the electromagnetic wave is coupled between a surface wave transmission line and a coaxial cable most often employ a horn-like surface wave "launcher" for forming the transition between the coaxial cable and the surface wave transmission line. In such a launcher, the surface wave transmission line forms an axial extension of the center conductor of the coaxial cable and a relatively thin-walled conductive horn in effect forms an outwardly flared extension of the outer conductor of the cable. That is, the smaller end of the horn, which is electrically connected to the outer conductor of the coaxial cable, generally is equal in diameter to the coaxial cable outer conductor with the diameter of the horn increasing as a function of distance measured from the interface with the coaxial cable toward the circular opening that is formed at the distal end of the horn.
Various attempts have been made in the prior art to smoothly contour the inner surface of a launcher horn to provide efficient coupling of energy between a coaxial cable and a surface wave transmission line. For example, U.S. Pat. No. 2,852,753 discloses a surface wave launcher wherein the inner wall of the launcher horn includes a throat region that extends between the interface of a surface wave transmission line and a coaxial cable and a bell region that extends from the terminus of the throat region to the end or mouth of the horn. In this arrangement, the inner surfaces of the throat and bell regions merge smoothly into one another, with each region being contoured so that the first three derivatives of the mathematical formula that define the inner diameter of the horn in terms of axial distance are each equal to zero when the distance variable is equal to zero (i.e., when the first three derivatives are evaluated at the interface between the coaxial cable and the launcher). The two specific examples of mathematical formulas that are disclosed in the referenced patent include: D=d (cosh Kx+cos K x)/2 and D.sup.4 =d.sup.4 +K.sup.4 X.sup.4, where D represents the inner diameter of the horn, d represents the inner diameter of the coaxial cable outer conductor, K is a constant that is selected to provide the desired diameter at the mouth of the horn for a given axial length, and x represents axial distance along the horn as measured from the interface between the horn and coaxial cable.
Although launchers configured in accordance with the referenced patent and similar launchers in which the diameter of the horn increases linearly as a function of distance provide satisfactory operation in some situations, several disadvantages and drawbacks can be encountered. For example, although such prior art surface wave launchers may adequately match the impedance of the surface wave transmission line to the impedance of the coaxial cable over a band of frequencies, the impedance match is not sufficient to provide low-loss transmission in systems that must exhibit a transmission bandwidth on the order of one to four octaves. Further, some transmission systems impose dimensional constraints on the length and diameter of surface wave launchers that cannot be met by prior art arrangements without making unsatisfactory sacrifices in the form of relatively high transmission loss.